Motor vehicles with an electric drive train are equipped with a DC power grid, which distributes the electrical energy from a traction energy store (for example, a drive battery or a fuel cell) to electrical units, specifically to the electric drive unit of the drive train and auxiliary units of the motor vehicle. In general, the electrical units are not supplied with the DC voltage from the DC power grid, but with an alternating current, a multi-phase alternating current, a pulsed DC voltage or a DC voltage with a rating which differs from that of the DC power grid. Accordingly, power converters are arranged between the DC power grid and the respective units. The function of the power converters is the conversion of direct current from the DC power grid into the respective current profile, and vice versa, for example for the recovery of kinetic energy of the motor vehicle. Accordingly, coupled DC and AC power grids are present.
As a result of the high power demand of the electric drive system, the DC power grid and the AC power grids supplied by the latter are frequently configured in the form of a high-voltage network (for example, with voltages between 300 volts and 1000 volts) or IT (from the French “Isolé Terre” or English “insulated ground”) network. In order to ensure electrical safety, particularly for vehicle passengers, during operation, the insulation resistance of the electric power grid in relation to electrical ground is monitored. If the insulation resistance falls as a result of insulation faults, a motor vehicle user can be alerted, for example, so that measures for the repair of the power grid can be implemented promptly.
Conventionally, the insulation monitoring unit is electrically connected to the DC power grid of the motor vehicle, for example within the drive battery. In the prior art, insulation monitoring devices which employ a passive measuring method are widespread, as these devices are very cost-effective. By this arrangement, the DC voltage source which is present on the DC power grid is used to drive a measuring current between the overall network and electrical ground. From the ensuing measuring current, the insulation resistance of the overall network in relation to electrical ground can be determined by a known method.
However, the passive measuring method linked to the DC power grid is inaccurate and, in many cases, systematically incorrect, as the value of the insulation resistance of the AC power grid coupled via the power converters is not correctly determined. The measuring current dictated by the insulation resistance of the AC power grid is reduced by the operation of the power converters, and is customarily changed in a temporally varying manner. Thus, the insulation resistance of the AC power grid measured using a conventional monitoring device from the DC power grid is generally significantly higher than the actual value of the insulation resistance. Consequently, there is a risk than an in-service insulation fault on the AC power grid will not be detected. In summary, conventional devices employing the passive measuring method on the DC power grid are not appropriate for the monitoring of galvanically-coupled AC power grids.
In comparison with passive measuring methods, active measuring methods, in which a time-coded voltage signal drives the measuring current, which is measured by a lock-in amplifier, are cost-intensive and, as a result of their complexity, more susceptible to errors.
Document DE 10 2010 054 413 A1 describes a method for the location of an insulation fault in a system which comprises a DC section and an AC section, incorporating an inverter. The inverter comprises power switches which, during a given measuring interval, are permanently open (freewheeling) and permanently closed (switched-through state), respectively. As such circuit states do not occur during the operation of the motor vehicle, and cannot be implemented without the restriction of vehicle operation, this method cannot be deployed during vehicle operation.